1. Field of the Invention
This invention relates to a pneumatic tire advantageously improving the resistance to uneven wear without sacrificing the wet performances, performances on snow and steering stability on dry road surface, particularly tires usable for use in passenger cars and small-size trucks.
2. Description of the Related Art
As this type of the conventional pneumatic tire attaching importance to the wet performances, steering stability and performances on snow, there are tires having a tread pattern as shown by a developed view in FIG. 6.
In this case, a pair of main grooves 42 extending linearly and continuously in a circumferential direction are arranged on a central part of a treading face 41 at a given interval in a widthwise direction of the treading face, and one circumferential groove 43 extending continuously in the circumferential direction is arranged at a position corresponding to 10% of a width W of the treading face from each end of the treading face. In each half part of a central region 44 defined between the circumferential grooves 43 are arranged a plurality of curvedly slant grooves 45 at intervals in the circumferential direction, in which each of the curvedly slant grooves 45 extends from the circumferential groove 43 toward the main groove 42 and open-mouthedly terminates at the main groove 42 and an angle at an acute angle side with respect to the circumferential direction is gradually made small toward the main groove 42, whereby a tapered land portion 46 gradually decreasing a width toward the main groove 42 is defined between the circumferentially adjoining curvedly slant grooves 45. Also, a land row consisting of the tapered land portions 46 is formed in the each half part of the central region in a point-symmetry with respect to a center C of the treading face. Furthermore, transverse grooves 48 each inclining in the same direction as the curvedly slant groove 45 and opening at the circumferential groove 43 and the tread face end and having an angle at the acute angle side with respect to the circumferential direction larger than that of the curvedly slant groove 45 are formed in each side region 47 defined between the circumferential groove 43 and the tread face end at intervals in the circumferential direction, whereby a land in the side region 47 is rendered into a block row. Moreover, the “width” of the tapered land portion 46 means a land portion width measured in a direction perpendicular to a center line 1 of the land portion 46 extending in the extending direction thereof at a developed plan view of the land portion 46.
In each of the tapered land portions 46 is arranged a sub-groove 49 opening at one end to the circumferential groove 43 and terminating at the other end in the land portion 46 so as to extend on the center line 1 of the land portion 46 at a position corresponding to a middle between the mutually adjoining curvedly slant grooves 45, and a plurality of long sipes 50 extending from the sub-groove 49 and extending substantially in parallel along the sub-groove 49 and a plurality of short sipes 51 extending in a direction substantially perpendicular to the curvedly slant groove 45, whereby a plurality of sub-blocks 52 having a curved rectangular form with a long side in the extending direction of the long sipe 50 are formed together with the slant grooves 45 and the circumferential groove 43 in the tapered land portion 46. In each of the blocks in the side region 47 are formed a plurality of sipes 53 extending along the transverse groove 48 at a full width of the block to define sub-blocks 54 having a rectangular form with a long side in the extending direction of the sipe 53. Furthermore, a plurality of slant sipes 56 linearly extending upwards to the right in the drawing and opening to both the main grooves 42 are formed in a rib 55 between the main grooves 42 at even intervals in the circumferential direction.
According to the tire having the above construction, the wet drainage property is ensured by the grooves 42, 43, 45, 48 and 49, and the performances on snow are ensured by the curvedly slant grooves 45, transverse grooves 48 and sub-grooves 49, while edge components of the grooves 45, 48 and 49 and the sipes 50, 51, 53 and 56 extending in the widthwise direction of the treading face can develop a function of cutting water film on a wet road surface and contribute to more improvement of traction and braking performances on snow.
In the above conventional technique, however, there is a problem that since each of the sub-blocks 52 formed in the tapered land portion 46 and the sub-blocks 54 formed in the side block has a long rectangular form in the extending directions of the long sipe 50 and the sipe 53, if a force in a short-side direction is applied to these sub-blocks 52, 54 during the running of the tire under loading, a large fall-down deformation is caused in the sub-block to generate a large uneven wearing in these sub-blocks and hence the tapered land portions 46 and side block.